Acoustic liners are designed to suppress noise generated at particular frequencies. A typical single degree of freedom liner includes a sheet of honeycomb core with a solid backface and one or more porous facesheets. Some liner configurations include multiple core layers and various additional bulk absorbers. Acoustic impedance is composed of two parts, resistance and reactance. The acoustic impedance characteristics of a liner can be used as an assessment of the noise attenuation properties of the liner.
In the aircraft industry, acoustic liners are commonly used to attenuate noise in aircraft engines, in machinery used in aircraft production and maintenance facilities, and at airport buildings. In many aerospace applications, it is necessary to test an acoustic liner after it is installed at a permanent location, and to continue testing and monitoring the liner at different points in time. This ensures that the liner continues to operate as expected and continues to meet regulatory noise emission standards. Such on-site testing requires a portable acoustic testing device to be taken to the installation site and used on the liner to check for the acoustic output of the part, or in some instances, of an entire assembly having one or more acoustic liner components.
U.S. Pat. No. 5,684,251 describes a portable acoustic testing device that uses a digital audio tape player, an amplifier, a test head, an audio recorder, a battery power supply, and a spectral analyzer. The tape player provides an output test signal which is amplified and sent to the test head. The test head includes an acoustic signal generator and a waveguide that is positionable adjacent the liner being tested. The test head uses the test signal to elicit an acoustic response from the liner. A number of pressure transducers, or microphones, measure the liner response and send the information to the recorder for storage. The recorder then provides the information to the spectral analyzer which performs the desired acoustic analyses on the recorded acoustic response data signals.
The '251 device has a number of drawbacks. One disadvantage is that it is cumbersome to use due to the need to record the response data signals and then feed them into the signal analyzer for analysis. Another problem is that the microphones are frequently uncalibrated and are effected differently by variations in the system. This can cause errors in the final test results. Another disadvantage is that the test head used in the '251 device does not work well with all shapes and sizes of liners. In particular, for flat liner surfaces, the cupped end of the test head can be a hindrance to properly fitting the test head against the liner during, testing. In addition, the '251 device does not appear to be capable of testing small liner sections, such as noninstalled liner plug samples that are smaller in size than the '251 test head output end. Lastly, the '251 device does not appear to adequately protect a worker from accidental exposure to excessive noise levels during use. The '251 device has two switches that must be pressed in order to activate the '251 device. Although this is helpful, it does not ensure that the test head itself is properly positioned, i.e., the switches may be accidentally pressed when the test head is not positioned adjacent a liner.
Thus, a need exists for an improved portable acoustic measurement system. The ideal system would directly analyze test result signals and would ensure that both microphones are properly calibrated so that the affect of system components on the measured data are accounted for. In addition, the system would benefit from having a test head that could be easily used on all sizes and shapes of acoustic materials, including non-installed test plug samples. Further, the ideal system would provide additional noise safety features to avoid any inadvertent sounding of the test head. The present invention is directed to fulfilling these and other needs as described below.